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Old 18th September 2010, 09:23 PM   #11
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Thanks everyone, for sharing your favourite valve types with me!

I have two related questions about the 1/f noise:

1. Is there any relation between the equivalent input 1/f noise and the biasing conditions of the valve? If so, how should I bias a valve to get the smallest equivalent input 1/f noise?

2. Do you know of any long-term degradation effects that aggravate 1/f noise in old but unused valves? The reason why I'm asking is that in the article I referred to, a new EF86 was exactly on spec while three new old stock EF86's were far above the specified noise level.
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Old 18th September 2010, 09:52 PM   #12
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I keep some 11J7 tubes for a super-duper microphone preamps. They are especially low noise versions of 6J7 designed for AC filament power in very low noise devices for military and government usage.
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Old 19th September 2010, 12:48 AM   #13
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I recently "discovered" the 6J7 and can at least attest to the fact that is both quiet and good sounding. I'm using it in a power amplifier design, it is somewhat microphonic, but quiet on AC filament power.
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Old 19th September 2010, 01:40 AM   #14
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Quote:
Originally Posted by kevinkr View Post
I recently "discovered" the 6J7 and can at least attest to the fact that is both quiet and good sounding. I'm using it in a power amplifier design, it is somewhat microphonic, but quiet on AC filament power.
For low microphonic I have some other missile-rocket-science tubes. I won't name them, because I am going to buy supply for myself, then reveal all details.
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Old 4th October 2010, 08:11 PM   #15
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About the bias point dependence of the 1/f noise: I've borrowed the book "Noise" by Aldert van der Ziel from the Delft university library. Apparently there are several different mechanisms leading to 1/f noise in valves, all with their own bias dependence:

1. One of the 1/f noise mechanisms mentioned in the book is work function variations of the cathode due to atoms moving around. Ideally this gives a bias point independent equivalent input 1/f noise voltage.

2. If the cathode suffers from island effect, however, the 1/f noise due to work function variations gets worse at too low bias currents. As a rule of thumb, use at least 0.5 mA.

3. Another effect mentioned in the book is a theory about emission centres with a limited lifetime that Schottky came up with in 1926. This effect would give a bias current independent equivalent input 1/f noise voltage.

4. However, for oxide coated cathodes, 1/f noise due to variations in the conductance of the cathode coating give an equivalent input 1/f noise voltage that increases with increasing bias current. This effect gets much worse at low cathode temperatures.

5. Also 1/f noise due to a barium orthosilicate interface layer between cathode and coating gives an equivalent input 1/f noise voltage that increases with the bias current. Such an interface layer forms in aged valves that have not conducted much current.

The effect can be quite dramatic: a graph in the book shows an equivalent noise resistance of 70 kohm at 600 uA and 100 Hz for a fresh 6SJ7 valve and 6 Mohm for an aged 6SJ7. Maybe this is the reason why the new old stock EF86's in Bloehbaum's article were so much worse than the new EF86.

The interface layer can be removed by running the valve at a higher cathode temperature or a larger current over a prolonged period. By the way, the datasheets of some special quality valves specifically state that these do not suffer from the formation of an interface layer.

6. Some bright emitting valves with tungsten cathodes suffer from extra 1/f noise due to the emission of positive ions from the cathode. This effect is at it worst in the space charge region of the operating characteristic.

7. Finally, from an article abstract I found on the internet I understood that there is such a thing as partition 1/f noise, so using triodes or triode connected pentodes also helps against some of the 1/f noise.


To make a long story short, use triodes or triode-connected valves (or Frank Bloehbaum's patented circuit), do not use a filament voltage below the specified value and determine experimentally what bias current results in minimum 1/f noise. You may get a different result for an old valve than for a new valve of the same type.
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Old 4th October 2010, 08:36 PM   #16
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Very interesting. It is a pity that much of this stuff is still not easily accessible on the web. It is either in old books, or charged-for journals. When I worked in a university I could access some of this, but not now.

My guess is that partition 1/f noise is small, as I think it was not discovered until the 1960s. If it was large it would have been found earlier.
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Old 25th October 2010, 04:41 PM   #17
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Default High gain low noise stage ??

Hello , new player interested by your concern here
Surprisingly nobody's talking about cascode ??
I remember very efficient E88CC playing that game !!
But it was compared only against EF86 and anything else ...

Best regards , Patrick.
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Old 25th October 2010, 06:35 PM   #18
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Quote:
Originally Posted by MarcelvdG View Post
The article I referred to is about a trick to get rid of the partition noise in pentodes, but my question is more general: do you know any valve, triode, tetrode, pentode, whatever, that combines low noise with low hum and low microphony?

Regards,
Marcel
I have used:

Sovtek 6SL7
Sovtek 6SN7
Westinghouse JAN 6J5
6FQ7 (NOS, GE)
Motorola 6BQ7

All of these types proved to be very low in the microphony department, and hum-free.

The main problem with microphony occurred with Sylvania 6BQ7s with the series connected heaters. These rang like bells. The Motorola and Sylvania 6BQ7s with parallel connected heaters are not microphonic at all.

There is also a problem with some NOS 6SN7s, in that some of these can be nastily microphonic.

Quote:
Surprisingly nobody's talking about cascode ??
I've used a cascoded LTP design to eliminate an additional gain stage. It works just great, low microphonics and hum if you use the right VTs (parallel heater 6BQ7s, as opposed to the ones with series heater connections). Sounds excellent.
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Last edited by Miles Prower; 25th October 2010 at 06:41 PM.
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Old 27th October 2010, 09:26 PM   #19
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For whoever is interested: I've done some noise measurements on four triode-connected EF86's:

Two EF86's of the brand Amperex, bought second-hand on Koninginnedag (Dutch national holiday), complete box with all kinds of valves for one euro with no guarantee of any kind

Two EF86's of the brand Trigon, bought at a shop (Radio Service Twenthe in The Hague)

For all four I listened to the noise while checking the A-weighted noise by reading off quasi peak-peak values with an oscilloscope with and without 47 kohm//1 Mohm connected between grid and ground.

The first Amperex EF86 showed very erratic behaviour: very strong low-frequency noise that sounded like a broken contact below a certain bias current level (of the order of 1 mA), reasonably normal sounding noise at higher current, grid currents up to 1 uA flowing out of the valve.

The second Amperex EF86 had no measurable grid current, but it also had strong low-frequency noise that sounded like a broken contact below a certain bias current level, reasonably normal sounding noise at higher current (1 mA or higher).

The Trigon devices were more well-behaved: the noise sounded normal at all current levels I tried.

For all four EF86's the A-weighted equivalent input noise voltage was at its lowest at the highest current I tried: 4 mA anode current.

For the second Trigon EF86 I did more accurate noise measurements. Using a home-made 100 times amplifier with A-weighting filter and a laptop, I recorded the noise of the valve, of valve plus 47 kohm//1 Mohm between grid and ground, and only of the 100 times amplifier and computer. Using GoldWave, I applied a comb filter and a steep 20 Hz high-pass to get rid of any remaining hum and subsonic supply voltage variations. I then measured the RMS values (using Volume Match in GoldWave) and calculated the equivalent input noise from the results.

The results are (expressed as an equivalent white input noise voltage that would give the same integrated noise):

Second Trigon EF86:
flat (except for comb) 20 Hz to 20 kHz:
20.02 nV/sqrt(Hz) at 298 uA
9.15 nV/sqrt(Hz) at 907 uA
7.41 nV/sqrt(Hz) at 2073 uA
7.7 nV/sqrt(Hz) at 3960 uA

A-weighted (plus comb):
17.32 nV/sqrt(Hz) at 298 uA
8.82 nV/sqrt(Hz) at 907 uA
6.53 nV/sqrt(Hz) at 2073 uA
6.34 nV/sqrt(Hz) at 3960 uA

Bandpass 1 kHz to 1.2 kHz (plus comb):
9.89 nV/sqrt(Hz) at 907 uA
9.1 nV/sqrt(Hz) at 2073 uA
9.65 nV/sqrt(Hz) at 3960 uA

A-weighted noise for three valves measured with the inaccurate quasi peak-peak method:
Second Amperex EF86, the one without grid current:
228 uA: infinite (that is, valve noise is so high that the effect of the 47 kohm//1 Mohm is not visible)
1 mA: 8.26 nV/sqrt(Hz)
4 mA: 6.19 nV/sqrt(Hz)

First Trigon EF86:
220 uA: 12.08 nV/sqrt(Hz)
362 uA: 11.58 nV/sqrt(Hz)
2.16 mA: 6.44 nV/sqrt(Hz)
4.05 mA: 6.26 nV/sqrt(Hz)

Second Trigon EF86:
201 uA: 16.21 nV/sqrt(Hz)
347 uA: 14.76 nV/sqrt(Hz)
2.11 mA: 8.76 nV/sqrt(Hz)
3.89 mA: 7.64 nV/sqrt(Hz)
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Old 28th October 2010, 11:02 AM   #20
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Sorry, could you clarify these results? Are these with or without 1M, or the difference between the 47K readings and 1M readings? A 1M resistor would give 126nV/sqrt(Hz) of thermal noise on its own; a 47K 27nV/sqrt(Hz). Or did you have 47K in parallel with 1M? Have you subtracted off the thermal noise?

Although I am not clear exactly what you did, the results do show flicker noise at low frequencies getting worse at lower anode currents, while mid-frequency noise seems to be independent of current. Both results are surprising, although I am not clear what the contribution of grid current noise is.
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